Some technical problems can be solved by using only partially transparent glasses. These problems include the matting of glazings as well as the production of projection screens and other optical viewing items.
Usually one uses light refraction or light dispersion at phase interfaces such as occur, for instance, when particles of suitable dimensions are imbedded as the dispersing phase in a carrier phase and both phases have different refractive indices. The carrier phase preferably consists of appropriate synthetic resins. Aside from inorganic particles such as, for example, glass beads, industry has developed suitable polymer particles for imbedding in the carrier phase.
A polymerization process for manufacturing fine polymer particles is known through DE-OS No. 21 46 628. In this process, a monomer system (A) is disclosed which is capable of free radical addition polymerization and which contains 87 to 99.99% by weight of a monoethylenically unsaturated monomer and 0.01 to 3% by weight of at least one polyunsaturated monomer. This monomer system completely dissolves 5 to 50% by weight (as related to the entire mixture) of another soluble thermoplastic addition polymer in which the first polymer will be insoluble. The solution is subject to conditions in which the monomer system polymerizes by means of free radicals and forms basically spherical cross-linked polymer particles in which the particles have an average particle size of 0.5 to 30 microns and are dispersed in a continuous phase of the thermoplastic polymer.
According to this reference, the polymer particles are cross-linked and, therefore, are generally not subject to melting or distortion so that they cannot for instance, be pressed with powders or subjected to a jet-molding process.
In the form of a solution, the polymer can be sprayed onto acrylic resin plates. Possible uses of this polymer are as a matting lacquer and also as mixtures which are suitable for the production of viewing screens with revertive projection, in computers, in the television or film industry, and in photography.
Also known from DE-OS No. 21 46 607 are transparent to opaque mixtures of synthetic resins which contain spherical, cross-linked addition polymer particles containing 87 to 99.99% by weight of at least one monoethylenically unsaturated monomer and 0.01 to 3% by weight of a monomer cross-linking agent dispersed in a continuous phase of an addition polymer, whereby the addition polymer should be soluble in the mixed monomers. The mixture of synthetic materials preferably also contains an additional mixed polymerized cross-linking agent. Self-supporting, transparent plastic plates, plastic tracks and similar items are mentioned as areas of use. Matted plastics are obtained by spraying a solution of the polymer material onto acrylic sheets. The mixture is, moreover, supposed to be suitable for viewing screens with revertive projection.
Japanese JP-OS 80 93 102 (Chemical Abstracts 93, 240792y) describes light-dispersing acylic resin plates in sandwich configuration which are obtained by means of a continuous belt polymerization from an acrylic resin film and acylate monomers, which contain a light-dispersing agent.
From Japanese JP-OS No. 81 117 225 (Chemical Abstracts 96, 86627b), optical screens are known which have a broad light diffusion effect in one direction and, in the other direction, a lenticular effect.
These screens are produced by calendering pulverized glass fibers into modified acrylic resin with subsequent uniaxial stretching, followed by a final coating with an acrylic layer containing carbon black.
Light-dispersing acrylic resin plates as projection screens are the subject of JP-OS No. 82 05742 (Chemical Abstracts 96, 200768a) in which the desired light dispersion is achieved by small imbedded quartz beads with a diameter of 0.5-20 microns in PMMA. According to GB-PS No. 1 585 338 light-dispersing plates with better optical resolution, a larger optical half-value angle and greater flexibility than conventional glass fiber plates are obtained if one uses plates of synthetic materials which are filled with glass beads. The glass beads have a diameter of 0.001 to 0.05 mm. Thermoplastics such as cellulose ester, polycarbonate, polyarylsulfonate, polystyrene, or acrylic resin are mentioned as carrier materials.
Previously known solutions concerning transparent to opaque mixtures of synthetic resins and their use for producing plates or sheeting materials have not been found to be entirely satisfactory.
In particular, the requirements for optical projection screens have not been fully met. There continues, therefore, to be a need for a method of producing opaque plastics without significant technical disadvantages.